One known technology for controlling relays and releasing welded contacts autonomously using a microcomputer is configured as shown in FIG. 6. More specifically, reference numeral 21 is a microcomputer, reference numeral 21a is a +DC power supply VDD, and reference numeral 21b is a power supply VSS on a common line with the load power supply. A relay 22 is connected to a relay control output 21c of the microcomputer 21 through a driver transistor 23. A contact 24a of the relay 22 is connected to a power supply 26 through a load 25, and another contact 24b is connected to an input 21d of the microcomputer 21 to detect welding of the contact 24a.
Control operations when the contact is welded in the above configuration is briefly described below.
When the relay control output 21c of the microcomputer 21 switches from ON to OFF, the coil voltage of the relay 22 turns OFF, and the load 25 also turns OFF. At this point, if the relay 24a is welded, the contact 24a remains turned ON and the recovery signal from the contact 24b for detecting welded contacts will not return to the input 21d of the microcomputer 21, thus generating the message that welding has occurred. This switches the control signal from the relay control output 21c of the microcomputer 21 to the welded contact release mode, in which a short pulse signal is applied to the coil of the relay 22 to release the welded contacts. If the contacts separate immediately, the welding releasing mode returns to the normal control mode. If not, the welding releasing operation is repeated until the contact is released.
With the above conventional configuration, however, a rise time is typically required by the relay driving power supply before a subsequent pulse can be applied to release the welded contacts. This causes a lack of continuity in the release pulse, decreasing its effectiveness.
Moreover, if an instantaneous power failure occurs during normal operation of the relay and the driving power supply of the relay is regained without reaching a sufficient sensory voltage level, the relay contacts may remain turned off, or contact pressure may become insufficient. In the worst case, the contact may generate heat, resulting in degradation of the reliability of the entire piece of equipment.